The IPCC sea level numbers

The sea level rise numbers published in the new IPCC report (the Fourth Assessment Report, AR4) have already caused considerable confusion. Many media articles and weblogs suggested there is good news on the sea level issue, with future sea level rise expected to be a lot less compared to the previous IPCC report (the Third Assessment Report, TAR). Some articles reported that IPCC had reduced its sea level projection from 88 cm to 59 cm (35 inches to 23 inches) , some even said it was reduced from 88 cm to 43 cm (17 inches), and there were several other versions as well (see “Broad Irony”). These statements are not correct and the new range up to 59 cm is not the full story. Here I will try to clarify what IPCC actually said and how these numbers were derived. (But if you want to skip the details, you can go straight to the critique or the bottom line).

It is this table on which the often-cited range of 18 to 59 cm is based. The accompanying text reads:

• Model-based projections of global average sea level rise at the end of the 21st century (2090-2099) are shown in Table SPM-3. For each scenario, the midpoint of the range in Table SPM-3 is within 10% of the TAR model average for 2090-2099. The ranges are narrower than in the TAR mainly because of improved information about some uncertainties in the projected contributions15. {10.6}.

Footnote 15: TAR projections were made for 2100, whereas projections in this Report are for 2090-2099. The TAR would have had similar ranges to those in Table SPM-3 if it had treated the uncertainties in the same way.

• Models used to date do not include uncertainties in climate-carbon cycle feedback nor do they include the full effects of changes in ice sheet flow, because a basis in published literature is lacking. The projections include a contribution due to increased ice flow from Greenland and Antarctica at the rates observed for 1993-2003, but these flow rates could increase or decrease in the future. For example, if this contribution were to grow linearly with global average temperature change, the upper ranges of sea level rise for SRES scenarios shown in Table SPM-3 would increase by 0.1 m to 0.2 m. Larger values cannot be excluded, but understanding of these effects is too limited to assess their likelihood or provide a best estimate or an upper bound for sea level rise. {10.6}

• If radiative forcing were to be stabilized in 2100 at A1B levels, thermal expansion alone would lead to 0.3 to 0.8 m of sea level rise by 2300 (relative to 1980–1999). Thermal expansion would continue for many centuries, due to the time required to transport heat into the deep ocean. {10.7}

• Contraction of the Greenland ice sheet is projected to continue to contribute to sea level rise after 2100. Current models suggest ice mass losses increase with temperature more rapidly than gains due to precipitation and that the surface mass balance becomes negative at a global average warming (relative to pre-industrial values) in excess of 1.9 to 4.6°C. If a negative surface mass balance were sustained for millennia, that would lead to virtually complete elimination of the Greenland ice sheet and a resulting contribution to sea level rise of about 7 m. The corresponding future temperatures in Greenland are comparable to those inferred for the last interglacial period 125,000 years ago, when paleoclimatic information suggests reductions of polar land ice extent and 4 to 6 m of sea level rise. {6.4, 10.7}

• Dynamical processes related to ice flow not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing future sea level rise. Understanding of these processes is limited and there is no consensus on their magnitude. {4.6, 10.7}

• Current global model studies project that the Antarctic ice sheet will remain too cold for widespread surface melting and is expected to gain in mass due to increased snowfall. However, net loss of ice mass could occur if dynamical ice discharge dominates the ice sheet mass balance. {10.7}

• Both past and future anthropogenic carbon dioxide emissions will continue to contribute to warming and sea level rise for more than a millennium, due to the timescales required for removal of this gas from the atmosphere. {7.3, 10.3}

(The above quotes document everything the SPM says about future sea level rise. The numbers in wavy brackets refer to the chapters of the full report, to be released in May.)

What is included in these sea level numbers?

Let us have a look at how these numbers were derived. They are made up of four components: thermal expansion, glaciers and ice caps (those exclude the Greenland and Antarctic ice sheets), ice sheet surface mass balance, and ice sheet dynamical imbalance.

1. Thermal expansion (warmer ocean water takes up more space) is computed from coupled climate models. These include ocean circulation models and can thus estimate where and how fast the surface warming penetrates into the ocean depths.

2. The contribution from glaciers and ice caps (not including Greenland and Antarctica), on the other hand, is computed from a simple empirical formula linking global mean temperature to mass loss (equivalent to a rate of sea level rise), based on observed data from 1963 to 2003. This takes into account that glaciers slowly disappear and therefore stop contributing – the total amount of glacier ice left is actually only enough to raise sea level by 15-37 cm.

3. The contribution from the two major ice sheets is split into two parts. What is called surface mass balance refers simply to snowfall minus surface ablation (ablation is melting plus sublimation). This is computed from an ice sheet surface mass balance model, with the snowfall amounts and temperatures derived from a high-resolution atmospheric circulation model. This is not the same as the coupled models used for the IPCC temperature projections, so results from this model are scaled to mimic different coupled models and different climate scenarios. (A fine point: this surface mass balance does include some “slow” changes in ice flow, but this is a minor contribution.)

4. Finally, there is another way how ice sheets can contribute to sea level rise: rather than melting at the surface, they can start to flow more rapidly. This is in fact increasingly observed around the edges of Greenland and Antarctica in recent years: outlet glaciers and ice streams that drain the ice sheets have greatly accelerated their flow. Numerous processes contribute to this, including the removal of buttressing ice shelves (i.e., ice tongues floating on water but in places anchored on islands or underwater rocks) or the lubrication of the ice sheet base by meltwater trickling down from the surface through cracks. These processes cannot yet be properly modelled, but observations suggest that they have contributed 0 – 0.7 mm/year to sea level rise during the period 1993-2003. The projections in the table given above assume that this contribution simply remains constant until the end of this century.

As an example, take the A1FI scenario – this is the warmest and therefore defines the upper limits of the sea level range. The “best” estimates for this scenario are 28 cm for thermal expansion, 12 cm for glaciers and -3 cm for the ice sheet mass balance – note the IPCC still assumes that Antarctica gains more mass in this manner than Greenland loses. Added to this is a term according to (4) simply based on the assumption that the accelerated ice flow observed 1993-2003 remains constant ever after, adding another 3 cm by the year 2095. In total, this adds up to 40 cm, with an ice sheet contribution of zero. (Another fine point: This is slightly less than the central estimate of 43 cm for the A1FI scenario that was reported in the media, taken from earlier drafts of the SPM, because those 43 cm was not the sum of the individual best estimates for the different contributing factors, but rather it was the mid-point of the uncertainty range, which is slightly higher as some uncertainties are skewed towards high values.)

How do the new numbers compare to the previous report?

Sea level rise as observed (from Church and White 2006) shown in red up to the year 2001, together with the IPCC (2001) scenarios for 1990-2100. See second figure below for a zoom into the period of overlap.

The TAR showed sea level rise curves for a range of emission scenarios (shown in the Figure above together with the new observational record of Church and White 2006). The range was based on simulations with a simple model (the MAGICC model) tuned to mimic the behaviour of a range of different complex climate models (e.g. in terms of different climate sensitivities ranging from 1.7 to 4.2 ºC), combined with simple equations for the glacier and ice sheet mass balances (“degree-days scheme”). This model-based range is shown as the grey band (labelled “Several models all SRES envelope” in the original Figure 5 of the TAR SPM) and ranged from 21 to 70 cm, while the central estimate for each emission scenario is shown as a coloured dashed line. The largest central estimate of sea level rise is for the A1FI scenario (purple, 49 cm).
In addition, the dashed grey lines indicate additional uncertainty in ice sheet behaviour. These lines were labelled “All SRES envelope including land ice uncertainty” in the TAR SPM and extended the range up to 88 cm, adding 18 cm at the top end. One has to delve deeply into the appendix of Chapter 11 of the TAR to find out what these extra 18 cm entail: they include a “mass balance uncertainty” and an “ice dynamic uncertainty”, where the latter is simply assumed to be 10% of the total computed mass loss of the Greenland ice sheet. Note that such an ice dynamic uncertainty was only included for Greenland but not for Antarctica; instability of the West Antarctic Ice Sheet, a scenario considered “very unlikely” in the TAR, was explicitly not included in the upper limit of 88 cm.

As we mentioned in our post on the release of the SPM, it is apples and oranges to say that IPCC reduced the upper sea level limit from 88 cm to 59 cm, as the former included “ice dynamic uncertainty” (albeit only for Greenland, as rapid ice flow changes in Antarctica were considered too unlikely to bother at the time), while the latter discusses this ice flow uncertainty separately in the text, stating it could add 10 cm, 20 cm or even more to the 59 cm in the table.

So is it better to compare the model-based range 21 – 70 cm from the TAR to the 18 – 59 cm from the AR4? Even that is apples and oranges. For one, TAR cites the rise up to the year 2100, the AR4 up to the period 2090-2099, thus missing the last 5 years (or 5.5 years, but let’s not get too pedantic) of sea level rise. For 2095, the TAR projection reduces from 70 cm to 65 cm (the central estimate for A1FI reduces from 49 cm to 46 cm). Also, the TAR range is a 95% confidence interval, the AR4 range a narrower 90% confidence interval. Giving the TAR numbers also as 90% ranges shaves another 3 cm off the top end.

Sounds complicated? There are some more technical differences… but I will spare you those. The Paris IPCC meeting actually discussed the request from some delegates to provide a direct comparison of the AR4 and TAR numbers, but declined to do this in detail for being too complicated. The result was the two statements:

The TAR would have had similar ranges to those in Table SPM-3 if it had treated the uncertainties in the same way.

and

For each scenario, the midpoint of the range in Table SPM-3 is within 10% of the TAR model average for 2090-2099.

(In fact delegates were told by the IPCC authors in Paris that with the new AR4 models, the central estimate for each scenario is slightly higher that with the old models, if numbers are reported in a comparable manner.)

The bottom line is thus that the methods have significantly improved (which is the reason behind all those methodological changes), but the expectation of how much sea level will rise in the coming century has not significantly changed. The biggest change is that ice sheet dynamics look more uncertain now than at the time of the TAR, which is why this uncertainty is not included any more in the cited range but discussed separately in the text.

There’s a number of issues worth discussing about these sea level numbers.

The first is the treatment of potential rapid changes in ice flow (item 4 on the list above). The AR4 notes that the ice sheets have been losing mass recently (the analysis period is 1993-2003). Greenland has contributed +0.14 to +0.28 mm/year of sea level rise over this period, while for Antarctica the uncertainty range is -0.14 to +0.55 mm/year. It is noted that the mass loss of Antarctica is mostly or entirely due to recent changes in ice flow. The question then is: how much will this process contribute to future sea level rise? The honest answer is: we don’t know. As the SPM states, by the year 2095 it could be 10 cm. Or 20 cm. Or more. Or less.

The IPCC included one guess into the “model-based range” provided in the table: it took half of the Greenland mass loss and the whole Antarctic mass loss for 1993-2003, and assumed this would remain constant ever after until 2100. This assumption in my view has no scientific basis, as the ice-flow is almost certainly highly variable in time. The report itself states that this ice loss is due to a recent acceleration of flow, and that in 2005 it was already higher, and that in future the numbers could be several times higher – or they could be lower. Adding such an ill-founded number into the “model-based” range degrades the much more reliable estimates for thermal expansion, mountain glaciers and mass balance. Even worse: to numbers with error estimates, it adds a number without proper error estimate (the observational uncertainty for 1993-2003 is included, but who would claim this is an error estimation for future ice flow changes?). And then it presents only the combined error margins – you will notice that no central estimate is provided in the above table. If I had presented this as an error calculation in a first-semester physics assignment, I doubt I would have gotten away with it. The German delegation in Paris (of which I was a member) therefore suggested taking this ice-flow estimate out of the tabulated range. The numbers would have become slightly lower, but this approach would not have mixed up very different levels of uncertainty, and it would have been clear what is included in the table and what is not (namely ice flow changes), rather than attempting to partially include ice flow changes. The ice flow changes could have been discussed in the text – stating there that at the 1993-2003 rate, this term would contribute 3 cm by 2095, but it is bound to change and could turn out to be 10 cm or 20 cm or more. However, we found no support for this proposal, which would not have changed the science in any way but improved the clarity of presentation.

As it is now, because of the complex and opaque way of combining the errors, even I could not tell you by how much the upper limit of 59 cm would be reduced if the questionable ice flow estimate was taken out, and one of the reasons provided by the IPCC authors for not adopting our proposal was that the numbers could not be calculated quickly.

A second problem with the above range is that the models used to derive this projection significantly underestimate past sea level rise. We tried in vain to get this mentioned in the SPM, so you have to go to the main report to find this information. The AR4 states that for the period 1961-2003, the models on average give a rise of 1.2 mm/year, while the data show 1.8 mm/year, i.e. a 50% faster rise. This is despite using observed ice sheet mass loss (0.19 mm/year) in the “modelled” number in this comparison, otherwise the discrepancy would be even larger – the ice sheet models predict that the ice sheets gain mass due to global warming. The comparison looks somewhat better for the period 1993-2003, where the “models” give a rise of 2.6 mm/year while the data give 3.1 mm/year. But again the “models” estimate includes an observed ice sheet mass loss term of 0.41 mm/year whereas ice sheet models give a mass gain of 0.1 mm/year for this period; considering this, observed rise is again 50% faster than the best model estimate for this period. This underestimation carries over from the TAR models (see Rahmstorf et al. 2007 and the Figure below) – this is not surprising, since the new models give essentially the same results as the old models, as discussed above.

Comparison of the 2001 IPCC sea-level scenarios (starting in 1990) and observed data: the Church and White (2006) data based primarily on tide gauges (annual, red) and the satellite altimeter data (updated from Cazenave and Nerem 2004, 3-month data spacing, blue, up to mid-2006) are shown with their trend lines. Note that the observed sea level rise tends to follow the uppermost dashed line of the IPCC scenarios, namely the one “including land ice uncertainty”, see first Figure.

We therefore see that sea level appears to be rising about 50% faster than models suggest – consistently for the 1961-2003 and the 1993-2003 periods, and for the TAR models and the AR4 models. This could have a number of different reasons, and the discrepancy could be considered not significant given the error ranges of observations and models. It is no proof that models underestimate future sea level rise. But it is at least a plausible possibility that the models may underestimate future rise.

A third issue worth mentioning is that of carbon cycle feedback. The temperature projections provided in table SPM-3 of the Summary for Policy Makers range from 1.1 to 6.4 ºC warming and include carbon cycle feedback. The sea level range, however, is based on scenarios that exclude this feedback and thus only range up to 4.5 5.2 ºC. This could easily be misunderstood, as in table SPM-3 the temperature ranges including carbon cycle feedback are shown right next to the sea level ranges, but the latter actually apply to a smaller temperature range. As a rough estimate, I suggest that for a 6.4 ºC warming scenario, of the order of 20 15 cm would have to be added to the 59 cm defining the upper end of the sea level range.

A final point is the regional aspects. Planners of coastal defences need to be aware that sea level rise will not be the same everywhere. The AR4 shows a map of regional sea level changes, which shows that e.g. European coasts can expect a rise by 5-15 cm more than the global mean rise – that is a model average, not including an uncertainty range. The pattern in this map is remarkably similar to that expected from a slowdown in thermohaline circulation (see Levermann et al. 2005) so probably it is dominated by this effect. In addition, some land areas are rising and some are subsiding in response to the end of the last Ice Age or due to local anthropogenic processes (e.g. groundwater withdrawal), which local planners need to account for.

The main conclusion of this analysis is that sea level uncertainty is not smaller now than it was at the time of the TAR, and that quoting the 18-59 cm range of sea level rise, as many media articles have done, is not telling the full story. 59 cm is unfortunately not the “worst case”. It does not include the full ice sheet uncertainty, which could add 20 cm or even more. It does not cover the full “likely” temperature range given in the AR4 (up to 6.4 ºC) – correcting for that could again roughly add 20 15 cm. It does not account for the fact that past sea level rise is underestimated by the models for reasons that are unclear. Considering these issues, a sea level rise exceeding one metre can in my view by no means ruled out. In a completely different analysis, based only on a simple correlation of observed sea level rise and temperature, I came to a similar conclusion. As stated in that paper, my point here is not that I predict that sea level rise will be higher than IPCC suggests, or that the IPCC estimates for sea level are wrong in any way. My point is that in terms of a risk assessment, the uncertainty range that one needs to consider is in my view substantially larger than 18-59 cm.

A final thought: this discussion has all been about sea level rise until the year 2095. Sea level rise does not end there, as the quotes from the SPM at the beginning of this article show. Over several centuries, without serious mitigation efforts we may expect several meters of sea level rise. The Advisory Council on Global Change of the German government (disclosure: I’m a member of this body) in its recent special report on the oceans has proposed to limit long-term sea level rise to a maximum of one meter, as a guard-rail to guide climate policy. But that’s another story.

Update: I was just informed by one of the IPCC authors that the temperature scenarios without carbon cycle feedback range up to 5.2 ºC, not 4.5 ºC as I had assumed. This number is not found in the IPCC report; I had tried to interpret it from a graph, but not accurately enough. My apologies! The numbers in the text above that had to be corrected are marked by strikethrough font. -stefan

Since this is in centimetres could you juxtapose these figures with any level of uncertainty with the 20 feet rise should Greenland and the WAIS both melt? I watched Lomborg testify in Congress and this was the one point he made of Gore’s film where he claimed it was false beyond any doubt.

[Response: Greenland ice is good for 7 metres and the WAIS for 6 metres of sea level rise. 20 feet is about 6 metres so either ice sheet alone, or half of each, could lead to a 20 feet rise. -stefan]

[The main conclusion of this analysis is that sea level uncertainty is not smaller now than it was at the time of the TAR, and that quoting the 18-59 cm range of sea level rise, as many media articles have done, is not telling the full story. 59 cm is unfortunately not the “worst case”.]

Worse case, in my view, is not the high range of sea level rise….it is merely the fact the rise will continue rising and there is no possibility for the sea rise to retreat….not in centuries or longer.

Yesterday afternoon (3/26/07) I attended a public workshop on “Rapid Changes in Ice Sheet Mass Balance” organized by the US Climate Change Science Program. The people giving presentations, who know ice as well as anyone, made a clear case not only that we have little understanding of ice dynamics, but that (as one of them put it) the main sources of uncertainty in the models are all in the direction of underestimation of the sensitivity of ice sheets to a temperature rise. But then the IPCC didn’t model ice dynamics anyway, they just assumed that nothing different could happen. I think it would be hard to find anyone familiar with the subject who thinks the IPCC upper bounds are a good guide for policy makers… What were they thinking of?

[Response: One of the reasons this stuff wasn’t included in detail in the IPCC report is that it is all pretty new. Anythinig included in the report has to have stood the test of time, at least a bit. The rule was anything cited had to be in press by May 2006. Many of the important papers postdate that. All this goes to show that IPCC is for the most part, conservative. That’s how science works, contrary to what the “skeptics” claim.–eric]

C’mon you guys! This is funny.
Nobody can even predict who will win the NCAA B-Ball tournament with any reasonable certainty!
And now some number crunchers with dubious models and no real understanding yet as to how the climate really works are telling us what the climate will be like in a 100 years? There are so many events that could happen between now and then to change everything.
But hey, if we’re going to play this game, then here’s my prediction: global temperature will be 0.28°C colder than today and sea levels will be 93.7 mm higher. I’ll spare you my 5 other scenarios. Michael Crichton is right, an informed guess is just a guess.

Thank you very much Stefan for this clarification! I think the point you mentioned about the riskassessment ist very importand for people who have to decide about the security of costal homelands for the next centuries. They should always take into consideration the worst case scenario and not only what is allowed for policymakers to read. The next point I appreciate are the graphs you presented. If the measured data are really at the upper end of the past predictions – this should alert everybody who has to deal with these issues.

George Kukla states, that increasing temperature will lead to increased snow precipitation and snow gains will offset the sea level rise – he warns of coming ice age ;-). He apparently did not change his mind since 1970-ties…

Considering the carbon-cycle feedback, some models (e.g. Cox et al.) estimate large positive vegetation feedback (increased soil respiration, lower photosynthesis due to increased vegetation stress, increased fire frequency…) and some of the most extreme scenarios predict the CO2 concentration to be up to 980 ppm. This is what I call catastrophe…

Hmm – 6C rise in global temperature – i think we are in trouble even with the rise of 3C…

Mark Serreze in his latest article in Journal Science states, that complete summer ice melt in Arctica is increasingly probable – in fact it is only matter of time, when it will happen. We know, that Arctic is melting quite rapidly. Antarctica is melting much slower, if at all. My question is:

Do we know from from paleoclimatology, that we can have a climate state, in which there is *no* summer ice in the North pole, while the South pole “is ok”?

I think this would create a bit of thermal imbalance in the climate and also would mean large changes in the weather pattern, which would *not* be limited only in the arctic region…

I just wonder, what we tell our children, if they will ask what were we doing, when we were aware of this possibility…

Off topic comment, but I do not know where else to have this question asnswred (if anyone could kindly direct me, I would appreciate it). I’ve been chatting with a denialist, who insists that a big reason he does not “believe” in anthropogenic climate change is because “ice cores going back thousands of years cannot tell temperatures or CO2 levels when the temperature was too warm for ice formation.” ie. Perhaps a sudden, rapid spike in CO2 levels and global temperature like we are now experiencing hashappened in the past, naturally, but we are not aware of it because no ice cores exist to tell us. Now I know this is a stupid argument anyway (“a 30% increase in human CO2 and a corresponding rise in temperature are not linked, because we don’t know that it can’t happen naturally”) but I was wondering if he was at least right about the ice core data. Thanks!

For what it’s worth, I’ve had an insightful flash into how to stop global warming. According to Lilo, Pudge, a fish off the coast of Kauai, controls the weather. If we can keep him in peanut butter sandwiches, everything will be under control.

Re 3 (Nick Riley). What’s interesting is that this is presented as ‘something strange that needs explaining’ – both by NERC and the BBC (as linked). There is no suggestion that it could indicate global warming scares are exagerated. If this study had shown that the Arctic sea level was rising more rapidly than elsewhere, this would have been headlined as further dramatic proof of impending doom. Its true that id does not disprove AGW. But it supports the case that all pieces of ‘evidence’ need to be considered with caution.

“… to determine Arctic sea level change …. is not an easy feat, as the ice cover obstructs the view of part of the sea surface and affects the measurements in mixed ocean/sea-ice conditions. Thus considerable effort has been put in the separation of the radar returns from leads and from sea ice. Moreover, microwave radiometer measurements of wet tropospheric delay can not be used since those measurements are also affected by sea ice. The Arctic altimeter data were retracked using an OCOG retracking algorithm, and the diffuse returns from the leads and open ocean were combined with a host of instrumental corrections and geophysical models to determine instantaneous mean sea level….”

Re#2
Yes you are right!
I would like to know the “weight” of each decimeter sea level rise regarding the consequences for people living near or far from costal regions? I suppose the first decimeter will be within current safety margins (at least I hope so!) but what will happen if the sea level will rise to the next decimeter and so far? What are the critical margins for whom? Are there any assumptions about the ability of mankind to adapt to theses changes? Are there projections what each decimeter will cost? So if 10 cm will cost nothing, 20 cm will cost 1 billion and 30 cm will cost 100 billion then we should know something more about the weight of these measures and not only look at a scalar value.

Thanks for clearing up some of the confusion. This point actually came up at my defense last week, as some of my committee was not aware that the new sea-level estimates did not include dynamic effects from the ice sheets, and therefore could not be considered ‘lower’ than previous estimates. It points to the importance of future dynamical modeling of ice flow.

Thanks for diving into this topic. The clear danger in issuing an SPM document is that nobody ends up reading the full report, nor incorporating the subtle complexities of the science into policy planning (which sadly is still likely to be the case even after the full report is issued.)

One of the ironies of all this is that we almost daily discover the limits of our knowledge regarding these processes, and find that we have erred in our thinking always on the lower end of the range of possibilities; nature seems generally to move faster and further than we guess at first. And I’m not the first to notice this. In fairness, our science has little ground proofing of theory on climate change because this is our first time having actual field observations. That said, if our predictive science is somehow missing the trajectory of the changes then that failure ought to become a topic of discussion in itself at some point.

Though I know it is entirely nonsense, in the back of my mind I imagine waking up one morning and catching a headline in the paper stating that Greenland has experienced a sharp increase in icequakes, and getting to work and checking CNN and finding out that half the ice sheet is currently in motion and accelerating, heading for the sea, and the UN Security Council is sitting in emergency session, and Wall Street is in a panic, and then looking out the office window at San Francisco Bay and realizing, once again, that our science was behind nature, not ahead of it, and we didn’t know how far behind we’d fallen.

Like I said, it’s all nonsense. But I would be comforted if someone could explain to me exactly what is nonsensical about it.

Michael Crichton doesn’t know his guess from a hole in the ground. If the CO2 goes up as in a given scenario, and all else is equal, and the known feedbacks are in place, then the temperature will rise by so much with error bars. What is so hard about that?

[[“ice cores going back thousands of years cannot tell temperatures or CO2 levels when the temperature was too warm for ice formation.” ]]

The ice cores are from places where there is snow deposition every year. We don’t measure temperatures from the layers directly, we calculate it from proxies like the level of carbon dioxide in bubbles and the O16/O18 ratio.

[Response: Correcton: the level of carbon dioxide in bubbles is never used to infer temperature. But yes, the 18O/16O ratio is one of several methods that is used to infer temperature. And yes, snow falls even in “warm” years on the Greenland summit, and the Antarctic plateau.-eric]

“In terms of a risk assessment, the uncertainty range that one needs to consider is in my view substantially larger than 18-59 cm…[T]his discussion has all been about sea level rise until the year 2095. Sea level rise does not end there…”

Then if you add in enhanced hurricanes & floods (there was a hurricane-looking downpour over Houston on March 14th, the rain going round & round, & not moving eastward very fast)….then the situation is bad indeed.

I wonder if our state planners are aware of global warming & its impacts (I know the water planners are not, according to a recent news article).

Great post. But did you see that Roger Pielke on 7 February attacked you guys for “adding to the confusion” for stating that the new 59 cm should not be compared to the old 88 cm? Clearly he was wrong and just trying to score some cheap points at your expense, without bothering to check the facts properly. His strategy seems to be to attack others so that he can then present himself as “honest broker”.

Why do the folks talking about ice sheet melt, talk about “global temperature”, when what affects the ice melt/ice dynamics is the temperature adjacent to the ice? Why do we talk about a global temperature increase of 1.4 or 1.6 degrees when Greenland ice pokes up into a region where the temperature has increased much more, and the ice sheets on the Antarctic Peninsula are in an area where the temperature changes over the last few years are even greater? What is the effect of a warmer North Atlantic Drift current on Greenland? What is the effect of warmer Antarctic waters on Antarctic ice that is in direct contact with that warmer water?

My point is that seawater at -0.5C has a very different effect on fresh water ice than seawater at +0.5C, and yet that is only 1 degree of warming. Not all degrees of warming are equal, and therefore averages do not mean much. The real question is, “What is the heat content of the water in contact with the ice sheets of concern?” This is where we can get some real near term excitement in the field of ice dynamics.

Re #14: Andreas Mueller — The first decimeter is not free for anyone living along a flat coast. The low countries, England, Germany and especially Bangladesh come to mind, but this is hardly a complete list.

The obvious problems are storm surges and salt water intrusion into wells, but there may well be others I haven’t learned about…

>MSNBC, Newsweek article claims:
“scientists have never found this phenomenon worrisome. Until this year, when Ronald Kwok of NASA’s Jet Propulsion Laboratory rang the alarm. He’d noticed that in 2005….”

Yeah, right. People had been trying to tell us about this for a long time before 2005. Al Gore got the Navy’s archive declassified to let this info be studied in public — in the previous millenium.

It is wrong to blame the journalists for getting the story wrong when it took Stefan eight screenfuls of text and diagrams to explain what the IPCC really means! It was stupid of the IPCC to combine projections of sea level with temperature when they knew that the two were not directly linked. Did they really think that the public, journalists, and sceptics would read (or even understand) the small print in the sea level column stating “Model-based range excluding future rapid dynamical changes in ice flow”? Or was this obfustication done deliberately at the request of the Bush Administration?

Of course, most IPCC scientists don’t believe that rapid dynamical changes in ice flow can happen! They believe in the 19th century uniformitarian paradigm of Charles Lyell, where geological change is slow and steady and all is right with the world. But that is wrong. Geology changes abruptly using eruptions, earthquakes and tsunamis, not to mention anoxic events and mass extinctions.

Climate too is a dynamical system, in which positive feedbacks can dominate. When they do, they are short lived, because the system will continue to change until negative feedbacks take over, and the system is again stable. Once stable it will remain there, until a new shock knocks it into another abrupt change with positive feedbacks again dominating.

We think the climate is stable because we have lived in the Holocene, which is one of those stable states. Like all other interglacials it will end abruptly, but rather than returning to an ice age, this one will switch into a hothouse world because of anthropogenic greenhouse gases.

Re #10 where Nick Riley asks about http://www.nerc.ac.uk/press/briefings/2006/arctic.asp , the answer is that the Arctic sea ice is thinning and the fresh water from the melting ice is being replaced by denser saline sea water. This causes the small fall in sea level in the Arctic where it is happening. It means that the Arctic sea ice has continued to thin by 0.1 m per year since Rothrock et al. reported in 1999. Since it was 2 m thick in 1997, simple arithmetic shows it will all be gone by 2017 if not sooner.

Of course this conflicts with the estimates for an ice free Arctic in 2040, 2050, and 2060 quoted by the scientists. For writing this, will I recieve another ad hominem attack, where I am accused of arrogance for daring to criticise them, and of stating the obvious? For instance – [Response: I don’t usually resort to sarcasm in my original response, above, but Alastair McDonald’s comment must surely rank as one of the most impressive displays of know-it-all-ness I’ve seen yet on RealClimate. He not only knows the cause, but he knows the solution to all our global warming concerns. -eric]

Or will I be ignored? That is their normal technique when they have no answer, but they know that they are right!

Re #8: […ice cores going back thousands of years cannot tell temperatures or CO2 levels when the temperature was too warm for ice formation.]

That seems pretty easy. If you’re measuring CO2 levels in the ice, you have some carbon, so you can apply C14 dating to it, no? If you do that (or use other dating techniques, the explanation of which I’ll leave to those more knowledgeable), and you find that you don’t have any gaps in the record, then his objection is answered.

Re #5. Pierre, As is clear from his books, Michael Crichton does not see the value in being informed. As such, I do not trust his assessment (or that of any other anti-science hysteric) for much of anything.
And actually, in many cases, an informed expert on basketball can indeed pick the winner more often than not. What is more, start with the opinion of experts and let a large number of “fans” weigh in (by betting), and that is how Vegas makes much of its money. (And it’s also not too dissimilar from scientific consensus, although the “fans” being scientists themselves are more professionally trained.)
In any case, these are scientific predictions–and scientific predictions have a pretty good track record of being true. You really ought to learn about the process.
By quoting Crichton as an authority, all you do is diminish your own.

I see the problem now, and it is partly true that we cannot tell the temperture beyond 150,000 years ago in Greenland because there is no ice before that date. However, we have been able to go back 700,000 years in Antarctica. Since carbon dioxide is a well mixed gas, then the CO2 in the Antarctic cores tells us about the CO2 worldwide as far back as 700,000 years ago. And we can extrapolate the temperature from the Antarctic ice core to the rest of the world.

There is other evidence that can be used to calculate the temperatures and CO2 levels before the times recorded in the ice cores, and also during the times of the ice cores. It was ocean sediment cores that proved Milankovitch was right, and fossil beaches that supported that idea. The ice cores are used because they have been proved right by other supporting evidence, and they show that the supporting evidence can be used for times before that recorded by the ice cores.

Steve, the AGU abstract I linked in #13 is the study on which that BBC article is based.

All — when a news article gives a few clues (AGU, author’s names) Google Scholar will almost always find you the actual abstract, at least.

I quoted about half the abstract in #13 — it’s a _very_ tentative conclusion for the reasons they state there. Good, difficult work trying to tease useful information out of that sort of data set. An exercise.

How certain are we that Greenland and Antarctica will take many hundreds or thousands of years to melt, even under heavy (5C+) global warming? My intuition is that the abnormal warming of the poles will continue, so a 5C rise in global temperature would mean perhaps a 15C rise in polar temperatures, and that should be able to melt Greenland in short order. Ice sheets have strong positive feedbacks, so I wouldn’t be surprised if they melt faster and faster as time passes.

“Alaskan and immediately adjacent Canadian glaciers supply one of the largest measured glaciological contributions to global sea level rise (~0.14 mm yr-1, equivalent to new estimates from Greenland). Retreating tidewater glaciers dominate the Alaskan sea level
contribution due to their ability to efficiently transfer mass via iceberg calving [Arendt et al., 2002]. During retreat phase, a tidewater glacier may retreat on the order of 1-2 km yr-1 concurrent with dramatic increases in ice velocity…”

RE #5 “And now some number crunchers with dubious models and no real understanding yet as to how the climate really works are telling us what the climate will be like in a 100 years? There are so many events that could happen between now and then to change everything.”

And one event between now and 100 years from now is that people might start reducing their GHGs, unlikely as that may seem. So, what d’ya say, let’s all reduce our GHGs by 70+% and end GW. That’ll show them arrogant, money-grubbing science types whose right.

And BTW, who needs models to tell us that heat melts ice, though my Sunfrost frig seems to take forever to defrost. But once that ice gets to a certain point, big chunks just start dropping down, one after another, kaboom! kaboom!

This brings me to catastrophe theory, which might fit those rather sudden changes in ice. I don’t know anything about CT, except that some functions look like potato chips….discontinuous, abrupt shifts.

Since the first IPCC report in 1990 confidence expressed in the validity of the AGW hypothesis has increased with each successive report. The 1990 report stated that: “the observed (20th century temperature) increase could be largely due to… natural variability”. The 1995 report said: “the balance of the evidence suggests a discernible human influence on climate”. In 2001 it was claimed “there is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities” and the current report concludes says it is: “90% probable” that the recent warming is “due to the observed increase in anthropogenic greenhouse gas concentrations”. During this time global expenditure on climate change research has been estimated to exceed $50 billion. It is now claimed that the science is settled,the evidence for AGW is overwhelming and the debate is over. . Unfortunately, few people, including most scientists other than climatologists, seem to have any clear or consistent idea of what this powerful new evidence actually is.

It would be of great value to public understanding if a short list of perhaps five to ten of the most important new findings (since 1990) in support of AGW was available. I am not suggesting a detailed report, just an annotated list with a few links to key references. In view of overwhelming scientific evidence and consensus such a list should be a trivial task for the experts on this forum. Certainly it would provide a valuable clarification to the fog of claims and counterclaims in the public perception.

It seems to me that when one has strong reason to suspect that one does not understand the most important factor in a situation (ice dynamics here), then quoting a quantitative value and a quantitative error range for all the second order effects which one does understand is singularly pointless. The reality is that climatologists currently have no adequate basis for quantitative projections of 21st century sea level, and they should say so as the main conclusion of the sea level section, not in hard-to-follow footnotes. The real story for the change from the TAR to AR4 is that “ice dynamics turned out to be much more important and rapid than we realized, and now we don’t know what to tell you” That should have been said in some reasonably clear manner.

Stefan:
You say you couldn’t get your suggested changes made to the summary. Why was this? Was it due to “political” pressures or time constraints, were you out-voted in some way, outgunned by more senior scientists, etc? Any further explanation of how the IPCC comes to a final document would be greatly appreciated by many of us outsiders!

Re #32: Have you checked the average temperatures at the South Pole and on top of the Greenland Ice Sheet? I think that you will find that even a 5deg C increase in annual average temperature will still see average temperature way below zero at the South Pole, and probably the same in Greenland.

So tell me. If the average temperature is still below zero, how much ice will melt?

[Response: Ice sheets are in equilibrium in a stable climate exactly because ablation (melt, in simple terms) balances accumulation (i.e. snowfall). In the center, where it is very cold, they gain mass, while around the edges they lose an equal amount. The problem with warming is not that it starts to melt the ice on the summit of Greenland or at the south pole, but around the edges of the ice sheets.]

Second. Surely the issue relating to the mass of the ice sheet in the case of on-land ice sheets is precipitation as well as temperature. If temperatures are below zero, any precipitation will increase the ice volume. So precipitation must be a key factor. How much do we know about precipitation/temperature inter-relationships?

[Response: As stated in my article, precipitation changes used in the projections are taken from a high-resolution atmospheric model. This is what leads to the assumption that Antarctica will gain mass overall, due to increased precipitation. However, this has not happened so far – until now, Antarctica seems to be losing mass.]

Third. To assess the issue of rising sea levels, why not go to the coast when a spring high tide is acting. That will show you at least some of the effect.

[Response: The problem of sea level rise does not arise on a calm day, even at spring tide. It arises during severe storm surges, which become a lot more frequent. For example, a study for New York showed that what is a once-in-a-century flooding event (submerging subway stations etc.) now, whould occur every 3 years if sea level were just 1 meter higher. -stefan]

First, thanks for the article – sea level rise is quite a hot topic right now. Secondly this post was not generated by a scep-bot.

It’s understandable that you should focus on the upside of potential sea-level rise as that is what your models predict. However, to those of us who continue to wonder at the appropriateness of using models to predict climate, two of your comments stand out:

“We therefore see that sea level appears to be rising about 50% faster than models suggest..”

“A second problem with the above range is that the models used to derive this projection significantly underestimate past sea level rise.”

Models surely are of less value if they are not used to produce the most accurate prediction of a future event. If they are wrong or have not reflected past events then the relevant parameters should surely be adjusted so that future projections will be consistent with past reality. Are you saying that they are and weren’t? “Significant” and “50%” are pretty fancy amounts wrong.

If a model runs with a known bias, what is the value of that model? Further, if that bias only exists for you while the model builder (believes she) has it right, then that makes your model wrong. If the amount wrong a model can be (whether yours or hers) is 50% then you don’t need too many wrong models before you have one 100% wrong.

I will anticipate the response that model results lie within a range (the IPCC stated one of which you in fact dismiss as being useful). But you have stated that they are wrong by so large a factor that one has to query how they are used, what outputs are released and of course more crucially what recommendations in terms of public policy are made as a result.

The whole point behind a summary is to – well, summarize. I accept that the IPCC has to be conservative and it is as well not to include very recent findings. However, we now seem to have a situation where the published worst case is actually not as bad as the true worst case. Politicians and journalists aren’t going to read the full report: the IPCC should update the summary when the report is published or risk an unwarranted degree of complacancy.

What is the maximum sea level rise before the ocean boils? I assume that there would be no ice anywhere at that time. The problem I have is that I don’t know the average slope of the shore. The shore is not vertical. What average shore slope is assumed in the IPCC computations? Please at least give me enough basic numbers to calculate backwards to get your assumed sea shore slope. Please send your answer to my email address.
I conclude that I should not buy land within a few hundred feet of sea level. My very bad back of the envelope calculation says sea level cannot rise more than 800 feet before the ocean boils. If the ocean boils, we are all cooked anyway.

[Response: If you melt all ice on Earth, sea level would rise about 70 metres. Last time this happened was in the Eocene, about 40 million years ago, when climate was about 4 ºC warmer than now (see Fig. 3.1-1 here).
The IPCC does not assume an average shore slope, it gives vertical sea level rise. If you want to roughly see the effect on coastlines, you can do that interactively here. -stefan]

Due to the dynamics of ice being little understood why is it assumed by the IPCC (for one) that it is unlikely for Antartica or Greenland to melt significantly. Is it simply intertia or is there some other science such as thermodynamics telling us that they will not lose much ice between now and 2100?

In additional how much ice would these places need to lose in order to weaken the oceans thermohaline/conveyor systems around the world ?

The rate of ice decrease at these places is likely to stay relatively constant and hence not add to much to sea level rises because they are huge stores of ice that take millenia to weaken and wear down even with a large scale warming event taking place ?

For the past month or so, sea surface temperature anomalies in the areas between the Antarctic and New Zealand and, subsequently, the Pacific, have been strongly negative. This has happened before, recently. ‘Tracking’ the path of the anomalies indicates their source.
In addition, I note this new paper in GRL: ‘Rapid Freshening of Antarctic Bottom Water from the Indian and Pacific Oceans’; Rintoul, March 2007.
It seems clear that the SSTAs are a function of seasonal sea ice loss at the surface. Does the Rintoul paper suggest the possibility of large outflowing of freshwater from below the surface, perhaps from subglacial lakes or rivers?
Can you tell me if either of these phenomena are unusual and, if so, whether such processes are accurately reflected in current estimates of ice mass balance, or whether they, too, suggest an underestimate. The Rintoul paper, in particular, may be indicative of a process which has not, afaik, previously been considered.
Regards,

Stefan, would you care to comment on Hansen’s paper? It seems to me that he makes a good case for “more alarmism”, i.e. scientist must speak out and say that we should seriously consider a sea level rise >1 m on a century scale, because there is a non-negligible possibility of that happening. From your article I guess you would agree that there is a 1% possibility (say). Wouldn’t that justify raising the alarm?

I think there is another reason why we should. The reaction I see in newspaper articles etc is: ‘Oh, IPCC says it’s just going to be 20-60 cm by 2100 [and Al Gore is an alarmist for bringing up the possibility of >5 m rise]. Like there will be some magical cut off in 2100! Of course history does not end in 2100, so if we’re not going to see large sea level rise in the 21st century, that’s no comfort at all for our grandchildren.

I am aware of the “crying wolf problem”; this is a complicated issue, and I don’t want to advocate “all out alarmism” just now, but I would like to hear some opinions.

[Response: I did not know Hansen’s paper but read it just now. I fully agree with what he writes about “scientific reticence”, his words echo my own experience very well. In many IPCC discussions I have noticed a strange asymmetry: people were very concerned about possibly erring on the high side (e.g., the upper bound of sea level rise possibly being criticised as “alarmist”), and not very concerned about erring on the low side (or some even regarding this as a virtue of being “cautious”). How likely is it actually that the rate of sea level rise in this century would on average be only half of the rate currently observed, despite further warming? But this is what would have to happen to reach the lower end of the IPCC range, namely 18 cm. (Current observed rate is 3.1 mm/year according to IPCC, or 3.3 mm/year using the satellite data 1993-2006.) Nobody was very concerned that 18 cm is a rather implausibly low value, possibly related to the fact that the models used to produce it already greatly underestimate the past sea level rise. Imagine the reverse had happened: models that greatly overestimate past sea level rise and come up with some implausibly high sea level rise number. Would IPCC have simply published that, as they did with the 18 cm value? I very much doubt it. Giving a low value is considered “safe”, it requires no courage for sticking your neck out, while giving a high number is considered risky and alarmist. I don’t think we are doing our job properly if we apply double standards to “low” and “high” estimates in this way. We need to dispassionately look at all the evidence, regardless of what is politically convenient or risky. -stefan]

Stefan: Very informative post, thank you. I wanted to ask if you could give a citation for the study you mentioned in your response to post No. 39 about frequency of subway flooding in New York.

[Response: Rosenzweig, C and Solecki, W D (eds) (2001) Climate Change
and a Global City: The Potential Consequences of Climate
Variability and Change. Metro East Coast. Report for the US
Global Change Research Program. National Assessment of the
Potential Consequences of Climate Variability and Change for
the United States. Columbia Earth Institute, New York. ]

I find your figure comparing the IPCC 2001 SLR scenarios with observed SLR to be compelling. The question I hope the appropriate scientific community is addressing is why is sea level rising 50% faster than the modeling projected? What are the most likely reasons for this? Reading your post and following some of the literature on dynamical changes it is tempting to conclude that the difference may be due to acceleratded ice sheet flow in Greenland and Antarctica that is not accounted for in the models. But there could also be errors in the modeling of the melt rate of mountain glaciers, the rate of snowfall on Greenland and Antarctica, and the underdog, failure to account for significant net groundwater depletion.

It might be interesting to show observed SLR in comparison to projections of the 1st and 2nd IPCC projections as well.

“Contraction of the Greenland ice sheet is projected to continue to contribute to sea level rise after 2100. … The corresponding future temperatures in Greenland are comparable to those inferred for the last interglacial period 125,000 years ago, when paleoclimatic information suggests reductions of polar land ice extent and 4 to 6 m of sea level rise. {6.4, 10.7}”

Further interglacial sea level rise is probably inevitable irrespective of anthropogenic warming contributions. Why should warming during the current Holocene Interglacial be significantly different/less than during the previous Eemian Interglacial (MIS5e), when sea levels were, per the IPCC’s own comments, above modern levels or for that matter above the purported mid-Holocene highstand?

In fact, the literature is pretty clear that the better analog for the Holocene is actually MIS11 (Droxler), around 400 kya, when orbital eccentricity was quite low as it is today, resulting in a longer interglacial (Berger) with sea levels higher than during the Eemian.

Note that during both the Eemian and MIS11, CO2 levels were lower than they are today, which strongly suggests a polar warming/melting mechanism other than CO2. Perhaps this anthropogenic warming debate is an unfortunate distraction from what we should be really focusing on – preparing for higher interglacial warming and rising sea levels, period.

– The poles go through (nearly) 6 months of darkness each year. With no sunlight in the winter, and with the sun at a low angle in the sky in the summer, it is always going to be cold at the poles and any melting in the summer is going to freeze back in the winter;

– The average annual temperature at the south pole is -49.5C. The average annual temperature at the north pole is -25.0C. 5.0C of warming leaves the poles very cold.

– Sea level has been rising at 1mm to 3mm every since the large continental glaciers melted after the last ice age – ie for the last 9,000 years.

– The interior of Greenland is below sea level. This is important if one is thinking of all of the ice melting or one is thinking of the glaciers “sliding off” into the sea.

– The arctic is still experiencing “rebound” from the weight of the glaciers from the last ice age.

– The Minimum Sea Ice Extent in the arctic was lower in 1990 than in 2006 – ie the arctic ice summer (September) minimum has been more-or-less stable for 16 years.

[Response: I’m always suspicious when someone announces: now here come the facts! You’d have to give some references for some of your “facts”. Sea level has been rising for 9,000 years at the rate of 1-3 mm/yr? So in the Middle Ages it was 1-3 meters lower than now? A number of studies (interestingly, some looking at where the Romans built fish ponds and other structures connected to the sea) and the IPCC rule this out, concluding just the opposite: in the preceding millennia, there was not even remotely the rate of sea level rise that is observed for the last century, it is a modern phenomenon. And on the September arctic sea ice cover, NASA has a very different story. -stefan]